Default fields for datasources

The server needs to send default fields per datasource. If the user performs a query on a datasource, for which he has no fields selected, the default fields for that datasource are automatically activated.

This will also affect live datasources. Currently, all data types received by a live datasource are automatically activated as fields. With this change, we can activate the default fields for a live datasource, as soon as the user clicks the play button. Because of this the user will get more control on which fields are used for the live datasource, but the simplicity of simply activating the live datasource remains.